A light-emitting diode (LED) among semiconductor light-emitting devices is widely used for full color displays, traffic light apparatus, automotive equipment, and the like.
Recently, since a gallium nitride based semiconductor (InGaAIN) LED can be combined with fluorescent materials to emit white light for lightning applications, it attracts considerable attention. In addition, developments have been actively directed to a high light flux LED driven by a large electric current of more than several-hundred mA.
A method of manufacturing an LED of this kind will be explained below with reference to FIG. 11. By applying a metal organic chemical vapor deposition (MOCVD) method to sapphire substrate 101, buffer layer 102, n-type GaN contact layer 103, n-type AIGaN clad layer 104, multiple quantum well (MQW) active layer 105, p-type AIGaN clad layer 106 and p-type GaN contact layer 107 are formed on the (0001)C-plane of sapphire substrate 101 in that order.
Next, a reactive ion etching (RIE) is applied to remove portions from the multiple layers of p-type GaN contact layer 107 through n-type GaN contact layer 103 to expose an n-type electrode region of n-type GaN contact layer 103.
A p-type electrode 108 is formed on a flat upper surface of p-type GaN contact layer 107, while an n-type electrode 109 is formed on the n-type electrode region of contact layer 103, so that an LED is manufactured.
In general, magnesium (Mg) is used as a p-type impurity. It is well-known, however, that Mg has a relatively deep acceptor level in GaN crystal, and an inert acceptor effect by atom-like hydrogen. Thus, Mg added, p-type GaN contact layer 107 hardly obtains sufficient carrier concentration and becomes high in electric resistance.
In the LED manufactured in the above mentioned method, contact resistance between p-type GaN contact layer 107 and p-type electrode 108 increases, it is difficult for p-type GaN contact layer 107 to have a good ohmic contact with p-type electrode 108, and an operation voltage of the LED becomes high.
The conditions result in increase in generation of heat by the LED and bring about declines in performance and reliability.
In order to address these conditions, nitride based semiconductors and a method of manufacturing the same are well-known (see, for example, the description at page 6 and FIG. 3 of Japanese Patent Disclosure 2002-16312).
The method of manufacturing nitride based semiconductor devices disclosed in Japanese Patent Disclosure 2002-16312 will be briefly explained below with reference to FIG. 12. A sectional view of the nitride based semiconductor device (laser diode) is shown in FIG. 12. The same reference numerals are put on substantially the same components as of the conventional LED shown in FIG. 11 and their explanation is omitted here.
As shown in FIG. 12, a stripe-like uneven portion 110 (the uneven period of which is a several μm to several tens of μm, and the concave depth of which is several tens of nm to several hundreds of nm) is defined in this p-type GaN contact layer 107 and p-type electrode 108 is engaged with uneven portion 110.
In order to make uneven portion 110, a photomask made of silicon dioxide (SiO2) or the like is formed on the upper surface of p-type GaN contact layer 107, photoresist is coated on so processed contact layer 107 and photolithography is applied to such coated contact layer 107 to make a stripe pattern, and a reactive ion etching (RIE) is carried out for so treated surface of p-type GaN contact layer 107.
A saw-tooth structure in a cross-sectional view and a grid-like structure of crossed rectangular stripes in a cross-sectional view are shown in Japanese Patent Disclosure 2002-16312 as structures of uneven portion 110.
Since the electrode is provided on uneven portion 110 as set forth above, a contact area of p-type electrode 108 with p-type GaN contact layer 107 increases and its contact resistance reduces.
The nitride based semiconductor device and its manufacturing method disclosed in Patent Publication 1, however, are complicated in manufacturing process, take a substantially long time to manufacture, and result in increase in production cost because they need the photolithography process to make the stripe-like uneven portion and the RIE process.
Present photolithography technology is hard to use in making a minute uneven portion with a stripe period of less than several μm of the above described limit in order to increase a contact area or to decrease electric contact resistance.
Because of the reasons set forth above, it is difficult to lower the operation voltage of the LED and also impossible to secure a sufficient voltage for a high light flux LED driven by a large electric current of more than several-hundred mA.
The reliability of the LED is likely to be affected by possible contamination of silicon from SiO2 photomask, residue damages due to the RIE process, and most likely uneven stress from a molded resin in the stripe-like uneven portion.
As explained above, it is quite difficult for the manufacturing method of the nitride based semiconductor devices disclosed in Patent Publication 1 to provide those semiconductor devices with a p-type electrode of sufficiently low electric contact resistance.